As telecommunications bandwidth requirements continue to rise, electrical wires as a communication medium are pushed to their capacity. The limitations of electrical wires has resulted in a growing trend towards the use of optical fibers to facilitate voice and data communications.
Communication signals traversing a communication channel lose strength as the distance traveled increases. To increase the distance that the signal travels, the signal is regenerated at certain points along the communication channel. Electrical signals are often regenerated using repeaters. In contrast, optical signals are regenerated using optical amplifiers.
A common optical amplifier is known as an Erbium Doped Fiber Amplifier (EDFA). An EDFA comprises a section of fiber, which is doped with erbium. A high-powered beam of light is mixed with the input signal using a wavelength selective coupler. The mixed light is guided into the erbium-doped section of fiber. The high powered light beam excites the erbium ions to a higher state. The photons of the signal light receive energy from the erbium ions when the erbium ions return to a lower state, thereby amplifying the signal.
The gain of the output signal from an optical amplifier often varies with signal wavelength. For example, an optical amplifier such as a Raman amplifier may produce a higher gain in lower wavelengths and a lower gain in higher wavelengths. In theory, other another type of amplifier might produce a higher gain in higher wavelengths and lower gain in lower wavelengths. The gain produced by such amplifiers is known as “titled gain.” In other cases, the optical amplifier may produce a gain that varies more complexly with respect to the wavelength.